Weather and Climate Extremes in a Changing Climate Regions of Focus: North America, Hawaii, Caribbean, and U.S. Pacific Islands Observed Changes in Weather and Climate Extremes Convening Lead Author: Kenneth Kunkel, Univ. Ill. Urbana- 2 Champaign, Ill. State Water Survey Lead Authors: Peter Bromirski, Scripps Inst. Oceanography, UCSD; Harold Brooks, NOAA; Tereza Cavazos, Centro de Investigación Científica y de Educación Superior de Ensenada, Mexico; Arthur Doug- las, Creighton Univ.; David Easterling, NOAA; Kerry Emanuel, Mass. Inst. Tech.; Pavel Groisman, UCAR/NCDC; Greg Holland, NCAR; Thomas Knutson, NOAA; James Kossin, Univ. Wis., Madison, CIMSS; Paul Komar, Oreg. State Univ.; David Levinson, NOAA; Richard Smith, Univ. N.C., Chapel Hill Contributing Authors: Jonathan Allan, Oreg. Dept. Geology and Mineral Industries; Raymond Assel, NOAA; Stanley Changnon, Univ. CHAPTER Ill. Urbana-Champaign, Ill. State Water Survey; Jay Lawrimore, NOAA; Kam-biu Liu, La. State Univ., Baton Rouge; Thomas Peterson, NOAA KEY FINDINGS Observed Changes Long-term upward trends in the frequency of unusually warm nights, extreme precipitation episodes, and the length of the frost-free season, along with pronounced recent increases in the frequency of North Atlantic tropical cyclones (hurricanes), the length of the frost-free season, and extreme wave heights along the West Coast are notable changes in the North American climate record. • Most of North America is experiencing more unusually hot days. The number of warm spells has been increasing since 1950. However, the heat waves of the 1930s remain the most severe in the United States historical record back to 1895. • There are fewer unusually cold days during the last few decades. The last 10 years have seen a lower number of severe cold waves than for any other 10-year period in the historical record which dates back to 1895. There has been a decrease in the number of frost days and a lengthening of the frost- free season, particularly in the western part of North America. • Extreme precipitation episodes (heavy downpours) have become more frequent and more intense in recent decades than at any other time in the historical record, and account for a larger percentage of total precipitation. The most significant changes have occurred in most of the United States, northern Mexico, southeastern, northern and western Canada, and southern Alaska. • There are recent regional tendencies toward more severe droughts in the southwestern United States, parts of Canada and Alaska, and Mexico. • For much of the continental U.S. and southern Canada, the most severe droughts in the instrumental record occurred in the 1930s. While it is more meaningful to consider drought at the regional scale, there is no indication of an overall trend at the continental scale since 1895. In Mexico, the 1950s and 1994-present were the driest periods. • Atlantic tropical cyclone (hurricane) activity, as measured by both frequency and the Power Dissipation Index (which combines storm 35 The U.S. Climate Change Science Program Chapter 2 intensity, duration, and frequency) has increased. The increases are substantial since about 1970, and are likely substantial since the 1950s and 60s, in association with warming Atlantic sea surface temperatures. There is less confidence in data prior to about 1950. • There have been fluctuations in the number of tropical storms and hurricanes from decade to decade, and data uncertainty is larger in the early part of the record compared to the satellite era beginning in 1965. Even taking these factors into account, it is likely that the annual numbers of tropical storms, hurricanes, and major hurricanes in the North Atlantic have increased over the past 100 years, a time in which Atlantic sea surface temperatures also increased. • The evidence is less compelling for significant trends beginning in the late 1800s. The existing data for hurricane counts and one adjusted record of tropical storm counts both indicate no significant linear trends beginning from the mid- to late 1800s through 2005. In general, there is increasing uncertainty in the data as one proceeds back in time. • There is no evidence for a long-term increase in North American mainland land-falling hurricanes. • The hurricane Power Dissipation Index shows some increasing tendency in the western north Pacific since 1980. It has decreased since 1980 in the eastern Pacific, affecting the Mexican west coast and shipping lanes, but rainfall from near-coastal hurricanes has increased since 1949. • The balance of evidence suggests that there has been a northward shift in the tracks of strong low pressure systems (storms) in both the North Atlantic and North Pacific basins. There is a trend toward stronger intense low pressure systems in the North Pacific. • Increases in extreme wave height characteristics have been observed along the Pacific coast of North America during recent decades based on three decades of buoy data. These increases have been greatest in the Pacific Northwest, and are likely a reflection of changes in storm tracks. • There is evidence for an increase in extreme wave height characteristics in the Atlantic since the 1970s, associated with more frequent and more intense hurricanes. • Over the 20th century, there was considerable decade-to-decade variability in the frequency of snowstorms of six inches or more. Regional analyses suggest that there has been a decrease in snowstorms in the South and lower Midwest of the United States, and an increase in snowstorms in the upper Midwest and Northeast. This represents a northward shift in snowstorm occurrence, and this shift, combined with higher temperatures, is consistent with a decrease in snow cover extent over the United States. In northern Canada, there has also been an observed increase in heavy snow events (top 10% of storms) over the same time period. Changes in heavy snow events in southern Canada are dominated by decade-to-decade variability. • There is no indication of continental scale trends in episodes of freezing rain during the 20th century. • The data used to examine changes in the frequency and severity oftornadoes and severe thunderstorms are inadequate to make definitive statements about actual changes. • The pattern of changes of ice storms varies by region. 36 Weather and Climate Extremes in a Changing Climate Regions of Focus: North America, Hawaii, Caribbean, and U.S. Pacific Islands 2.1 BACKGROUND 2006 was the sec- ond hottest year on Weather and climate extremes exhibit sub- record in the con- stantial spatial variability. It is not unusual for terminous United severe drought and flooding to occur simultane- States (Arguez, ously in different parts of North America (e.g., 2007). The areas catastrophic flooding in the Mississippi River experiencing the basin and severe drought in the southeast United largest tempera- States during summer 1993). These reflect ture anomalies in- temporary shifts in large-scale circulation cluded the higher patterns that are an integral part of the climate latitudes of Canada system. The central goal of this chapter is to and Alaska. An- identify long-term shifts/trends in extremes and n u a l a v e r a g e to characterize the continental-scale patterns of temperature time such shifts. Such characterization requires data series for Canada, sets that are homogeneous, of adequate length, Mexico and the and with continental-scale coverage. Many United States all data sets meet these requirements for limited show substantial periods only. For temperature and precipitation, warming since the rather high quality data are available for the middle of the 20th conterminous United States back to the late centur y (Shein, 19th century. However, shorter data records are 2006). Since the available for parts of Canada, Alaska, Hawaii, record hot year Mexico, the Caribbean, and U.S. territories. of 1998, six of In practice, this limits true continental-scale the past ten years analyses of temperature and precipitation (1998-2007) have extremes to the middle part of the 20th century had annual average Figure 2.1 Changes in the percentage of days in a year onward. Other phenomena have similar limita- temperatures that above three thresholds for North America for daily high temperature (top) and daily low temperature (bottom) tions, and continental-scale characterizations fall in the hottest (Peterson et al., 2008). are generally limited to the last 50 to 60 years 10% of all years on or less, or must confront data homogeneity record for the U.S. issues which add uncertainty to the analysis. Urban warming is a potential concern when We consider all studies that are available, but in considering temperature trends. However, re- many cases these studies have to be interpreted cent work (e.g., Peterson et al., 2003; Easterling carefully because of these limitations. A variety et al., 1997) show that urban warming is only of statistical techniques are used in the studies a small part of the observed warming since the cited here. General information about statisti- late 1800s. cal methods along with several illustrative Annual average examples are given in Appendix A. Since 1950, the annual percent of days ex- temperature time ceeding the 90th, 95th, and 97.5 percentile series for Canada, 2.2 OBSERVED CHANGES AND thresholds2 for both maximum (hottest daytime VARIATIONS IN WEATHER AND highs) and minimum (warmest nighttime lows) Mexico, and the CLIMATE EXTREMES temperature have increased when averaged over United States all all of North America (Figure 2.1; Peterson et show substantial 2.2.1 Temperature Extremes al., 2008). The changes are greatest in the 90th warming since the Extreme temperatures do not always correlate percentile, increasing from about 10% of the middle of the 20th with average temperature, but they often change century. in tandem; thus, average temperature changes seasonal/annual U.S. temperature and precipitation rankings: “near-normal” is defined as within the provide a context for discussion of extremes.